1. Field of the Invention
The present invention relates to a length measuring device having a measuring slide, a linear guide in the form of a hollow profile, on which the measuring slide is mounted so it is externally displaceable, to be able to bring it into contact with an object to be measured with respect to its length, an inner slide, which is mounted so it is displaceable in the interior of the hollow profile, a magnet assembly, which magnetically couples the measuring slide and the inner slide so that the inner slide follows every movement of the measuring slide along the hollow profile, a measuring unit for measuring the position of the inner slide along the hollow profile, and a display, which is visible in the exterior of the hollow profile, of the length ascertained by the measuring unit in accordance with the measured position of the inner slide.
2. Discussion of the Prior Art
The invention is directed in particular to length measuring devices for measuring the physical height of persons. Such length measuring devices are also referred to as stadiometers. A typical such length measuring device has a measuring rod, which is implemented as a vertical linear guide for a measuring slide (head slide). The measuring rod is aligned vertically and is installed on a wall or fastened on a platform. To measure the physical height, the person steps in front of the measuring rod, after which the head slide is pushed down on the measuring rod until it comes into contact on the head of the person to be measured. A measuring scale having scale graduation lines is provided on the measuring rod. A read unit is in turn provided in the measuring slide, which registers the scale graduation lines during the displacement of the measuring slide on the measuring rod and therefore detects the incremental change of the position of the measuring slide. The absolute height of a scale graduation line can also be coded in the scale graduation lines, so that the height of the head slide can be determined using the read unit, which is then displayed on a display on the measuring slide.
Another type of a length measuring device for the physical height has an angled part, which is held by a person carrying out the measurement of the physical length. A leg of the angled part is held in contact on the head of the person to be measured. A second leg protrudes perpendicularly from this leg in contact on the head, wherein the angled part is held so that the second leg extends directed vertically towards the floor. A distance measuring unit having an ultrasonic transducer is provided in the second leg, which, from the runtime of an emitted ultrasound signal, which is reflected on the floor and returns to the ultrasonic transducer, determines the height of the first leg located in contact on the head of the person to be measured above the floor and therefore the physical height and displays it. A disadvantage of this type of length measuring device is that measurement inaccuracies can occur because the person carrying out the measurement does not hold the angled part exactly aligned so that the second leg is oriented precisely vertically on the floor. Furthermore, it is disadvantageous that changing environmental conditions (for example, dust or other contaminants in the air) or objects lying on the floor could corrupt the measurement.
A further type of a length measuring device has a platform, onto which the person to be measured steps, and a horizontal support fixedly suspended vertically above the person to be measured. A distance measuring unit based on ultrasound wave runtime is attached to the support, which is directed towards the head of the person to be measured, who is standing on the platform. The person to be measured wears a cap to ensure a well-defined reflection of the ultrasound waves from the upper side of the head. The physical length of the person to be measured may be derived from the distance of the upper side of the head of the person to the distance measuring unit, which is permanently installed horizontally above the person to be measured, from the difference of the suspension height of the distance measuring unit and the measured distance to the upper side of the head. This length measuring device is also susceptible to error, since the measurement can be corrupted by changing environmental conditions and interfering influences in the open measuring section between the ultrasonic transducer and the upper side of the head of the person to be measured.
A length measuring device is disclosed by WO 98/17974 A1. This length measuring device is not implemented as a length measuring device for persons. Rather, the position of a measuring slide is to be tracked. The length measuring device has a linear guide in the form of a hollow housing, on which the measuring slide is mounted so it is externally displaceable. In the interior of the housing, an inner slide is mounted so it is displaceable therein. A magnet assembly couples the measuring slide and the inner slide magnetically, so that the inner slide follows every movement of the measuring slide along the guide. The inner slide is in slip contact with a linear potentiometer to provide a voltage signal, which is proportional to a position of the inner slide. The position of the inner slide and therefore that of the measuring slide coupled thereto along the linear guide is derived from the potentiometer signal. Certain interfering factors such as smoke or dust, which do not penetrate into the interior of the housing, are reduced in their influence on the measurement precision by the measurement of the position of the inner slide in the interior of the housing. The potentiometer has slip contacts, which create abrasion and thus wear over time. To counteract this, high-quality materials must be used, which increases the production costs, of course. However, the wear can result in worsening of the measurement precision even under these conditions.